Mi Edg 11298
Mi Edg 11298
Mi Edg 11298
A DISSERTATION
Submitted in partial fulfillment of the
requirements for the award of the degree
of
MASTER OF TECHNOLOGY
in
MECHANICAL ENGINEERING
(With Specialization In Production & Industrial System Engineering)
Ac NO.........
By i`
r3l~Jb'
I hereby declare that the work which is being presented in this dissertation
DESIGN" in the partial fulfillment of requirements for the award of the degree of
carried out from August 2002 to February 2003, under the supervision of
The matter embodied in this dissertation has not been submitted by me for
Date: z7/n12cv3
\ dj
Place: ROORKEE VINAY KUMAR SINGH
CERTIFICATE
This is to certify that the above statement made by the candidate is correct
i
ACKNOWLEDGEMENT
I cannot forget to recall my heartiest regards, the ever ending heart felt stream
of tendering love, which my whole family bestowed upon me. It was the power of
materialize my dreams.
I owe it to all my friends and well wishers who made this endeavors
them. Last but not the least, thanks God for keeping me in best health during this
(VINAVKUMAR SINGH)
ai
ABSTRACT
from few grams to several tons, complicated shapes, precision parts etc. in a very
for its strength consideration. Efficient and effective design of riser and gating
economically. The optimum designing of riser and gating system have not been
fully understood till now. Although, much of the knowledge has been obtained
during last few decades, there still exist and inability to integrate the knowledge
gating system and its interrelationship with other factors like choke area, pouring
time, sprue and pouring basin dimensions, shape and dimension of riser etc. the
system for Al-7% Si alloy castings. C language is used to develop the software
which can predict the theoretical pouring time, riser and gating system
dimensions, check for turbulence, check for aspiration and finally gives the
N
percentage casting yield as a performance index for the rigging system design. The
system, riser shapes etc. Software is well working for different combinations of
iv
CONTENTS
Page No.
CANDIDATE'S DECLARATION 1
ACKNOWLEDGEMENT ii
ABSTRACT iii
NOMENCLATURE viii
LIST OF FIGURES xi
CHAPTER 1: INTRODUCTION 1
SYSTEM DESIGN
V
2.4.3 Design of Sprue C
2.4.6 Design of Ingate 39
2.4.7 Design of Runner 41
2.4.8 Sprue-Runner-Gating Ratio 43
CHAPTER 3: RISER DESIGN 47
3.1 RISERING TERMINOLOGY 47
3.2 DESIGN OF RISER 53
3.2.1 Chvorinov's method 54
3.2.2 Caine's Method 55
3.2.3 Modulus method 58
3.2.4 Naval Research Laboratory Method 63
3.3 FEEDING DISTANCE 70
CHAPTER 4: SOFTWARE DETAILS 75
4.1 COMPUTER LANGUAGE 75
4.2 COMPUTER UTILIZATION IN RIGGING 76
SYSTEMS DESIGN
4.3 STEPS INVOLVED IN PROGRAMMING 77
vi
CHAPTER 5: RESULTS AND DISCUSSION 85
CHAPTER 6: CONCLUSIONS 101
REFERENCES 103
vii
NOMENCLATURE
P Pressure kg/sq.mm
C Efficiency Factor
h Potential Head mm
viii
ht Height of Sprue mm
R Reynond's Number
W Weight of Casting kg
and Risers
M Casting Modulus mm
Mr Riser Modulus mm
Dr Diameter of Riser mm
Hr Height of Riser mm
and Riser
Solidification Time
Vrf Riser Volume based on cu.mm
x
LIST OF FIGURES
and aspiration
gating system
xi
3.3 Caine's equation 57
xii
LIST OF TABLES
of average thickness
of average thickness
gating systems
xiu
CHAPTER 1
INTRODUCTION
competition. In the broad language of engineers, useful engineering parts are made
from metals and its alloys by metal forming. Many ways of forming metals are
used for a given part, machining may be required to a greater or lesser degree. In
these methods, only the casting aspect of metals fabrication is considered. Casting
is one of the oldest manufacturing processes and even today, it is the first step in
manufacturing most of the products. It is estimated that castings are used in 90%
or more of all manufactured goods and in all capital goods machinery used in
manufacturing [1].
economical way, and to obtain complicated shapes with little machining if needed.
The demand for high precision casting parts continues to increase due to exacting
demands from the automotive and aero-industries. Much research has been
devoted toward process development for the production of high quality casting
goods at low costs. The manufacture of a part involves several steps, the first of
which is the design of the part itself, and the specification of the material to be
used. This information is passed to the method engineer, who will choose the
casting process, and then design the gating and risering system or rigging system
necessary to get the molten metal into all regions of the part so as to produce a
sound casting. Two major considerations in the casting design are the quality of
the final product and the yield of the casting, both of which heavily depend upon
being. The discovery of the casting process was probably around 3500 B.C. in
around 1500 B.C. Before that there is no evidence of any casting activity found in
China. Through India could be credited with the invention of crucible steel, not
much of iron founding was evident in India. It appears iron casting technology in
India has been use from the times of the invasion of Alexander the Great, around
300 B.C.
The shortest distance between raw material and a finished part is a casting.
being. Metal castings are produced by pouring molten metal into mold having the
desired shapes and approximate sizes of the casting and allowing it to freeze and
thus take the form of the mold. After solidification and cooling, the desired metal
F)
object is taken out from the mould by breaking the mould. This is the fastest and
often the economical method for obtaining a part of any desired composition.
Functional Advantages:
Beyond the rapidly emerging technologies that are keeping metal casting in
the forefront in the metal forming industry, castings possess many inherent
advantages that have long been accepted by the design engineer and metal parts
flexibility of any metal forming process. The casting process is ideal because it
rigidity obtainable by no other method of fabrication. The shape and size of the
part are primary considerations in design, and in this category, the possibilities of
metal castings are unsurpassed. The flexibility of cast metal design gives the
engineer wide scope in converting his ideas into an engineered part [1].
The freedom of design offered through the metal casting process allows the
(i) Design both internal and external contours independently to almost any
requirement.
(ii) Place metal in the exact location where it is needed for rigidity, wear,
3
For many years the production of sound casting was largely a matter of trail
and error. Although, a few general principles were well recognized. The approach
for gating, risering and chilling are mainly based on the foundry man's hard-won
aspects of the design and production of castings. This initiated the studies done by
computer to simulate the casting solidification which offers better risering and
gating system
metal to the mould cavity at the proper rate, without excessive temperature loss,
free from objectionable turbulence, entrapped gases, slag and dross [9]. If the
liquid metal is poured very slowly then the time taken to fill up the mould is rather
long and the solidification may start even before the mould has been completely
filled up. This can be avoided by using too much superheat, but then gas solubility
may cause a problem. On the other hand, if the liquid metal impinges on the
mould cavity with too high velocity, the mould surface may be eroded.
Risers are reservoirs of molten metal that are used to feed the casting
during solidification to compensate for the shrinkage. This shrinkage includes the
during solidification causes voids unless more molten metal can be fed to the
potential problem spots. Risers are designed to solidify last and to draw the
shrinkage voids put of the casting. Risers also serve as exits for gases and dross
4
entrapped in the metal and as pressure heads to feed the section. These can be
gating system. In order to achieve the best economical and functional design, the
software program can be used to repeat the design process by changing the values
Metal flow rate and the quality of flow, metal fluidity, proper evaluation of
modulus and aspiration should be considered in the gating system design. Besides
prevention of slag and dross by proper choking, suitable gating ratio must be
utilized to ensure quiet filling of the mould and a sound casting free from defect,
thus minimizing the fettling cost. The software for gating and risering is developed
in C language.
advanced technology with the application of computer in metal casting. The books
on casting by Flinn, R. A., Rosenthal, P.C., Taylor, H.F. etc. gave elaborate views
regarding the basic concepts associated with casting. The most recent approach to
riser and gating design is, with the aid of computer. The literature available in this
area is as follows:
which information about the casting shape is input using a coordinate system of
geometric input. This requires the user to first express the dimensions of the
5
casting in coordinate after picking an arbitrary zero. The output data includes a
Heine, R.W., and Uicker, J.J., [121 developed an overall information flow
plan for computer aided engineering of casting processing and design is revealed.
section modulus of the geometric sub-shapes comprising the design. The section
moduli are obtained using a minicomputer program and graphics tablet input from
the casting drawing. Volume and weight of the casting are obtained in the same
revealing directional freezing from casting, through riser contacts, to the riser
obtained with a main frame computer program is also presented. Risering by the
geometric technique with cylindrical or tapered risers for specific examples with
Rader, L.A., and Haines, D.R., [221 presented the use of computer
simulations in the casting design process have the potential of reducing costs and
to the top of partially solidified risers to provide heat and liquid metal. The heat
6
remelts the risers and the liquid metal compensates for metal shrinkage during
casting. Experimental castings and computer simulations were studied for the
influence of changes in riser insulation and exothermic material amount and time
and earlier application improved the integrity and minimized segregation in the
castings. The simulation showed a level or invariant point in the casting above
The predictability of this level would be useful in evaluating other riser and
casting variations.
Davis, K.G., and Magny, J.G., [7] described a new modulus based
used to generate the shrinkage cavity profile for a given riser, and the feeding
efficiency of the riser is then evaluated on the basis of the profile. Test castings
with simple shapes have been poured in mild steel to examine the effects on the
gating design, riser shape, and feeding aids. The tests were also used to establish
Edwards, J.O., and Sahoo, M., [8] clarified the concepts of pouring rate
and metal velocity. Various factors such as height of the ladle lip above the mold,
sprue height, type of sprue (parallel vs. tapered), sprue diameter, type of gating
7
system (pressurized vs. Non-pressurized) etc., affecting both the pouring rate and
metal velocity are discussed. Metal velocity in various parts of the system is
calculated, and a comparison of calculated and measured flow rates is made for a
Chen, C., and Lewis, R.L., [5] addressed the traditional trapezoid shaped
runner and gate is widely used in the foundry. It has been assumed to be the best
shape to trap dross or slag during the casting filling process. In this research, a
order to determine the effectiveness of the trap gate in entrapping slag. Results
indicated that the trap gate shape is considerably more effective than the
Jordan, Hill, and Piwonka, [16] presented that there have been many
successful computer programs for designing and positioning feeding systems for
castings, only a few have been developed for gating systems, and these have a
number of limitations. The reasons for this are explored by examining the
requirement for good gating system, the physical fundamentals underlying these
rigging systems as they are sometimes referred to, has been a very important task
in the manufacture of cast components and also presented a compilation of common rules
of thumb used by foundry experts and guidelines suggested by researchers for better
quality castings. From the rules given, it can be observed that the geometric features of
the casting, such as casting boundaries, thick regions and flow paths, are of primary
The primary objective of this work is to help in designing of riser and gating
system. Gating system design involves determining the dimensions of pouring basin,
sprue, choke area, runner etc. and riser design consist of riser shape and its dimensions.
In general the basic problem come across by designers is repetitive and tedious
calculations, so the chance of making errors, time consuming and the design output may
not be accurate. There is a basic need for a software that represents the knowledge and
In the present work, software has been developed using C programming language
to design the riser and gating system for simple casting shapes of aluminum alloy in
0
CHAPTER 2
The term gating or gating systems refers to all the passageways through
which metal enters a mould cavity. It thus mainly includes parts such as a pouring
basin, sprue, runner, and gates. The chief requisites of a gating system are:
(i) The metal should flow through the gating system with as little
turbulence as possible, so that mould gases and air will not be trapped in
the stream and the molding sand will not be washed away.
(ii) The metal should enter the mould cavity in a manner that will produce
(iii) The gating system must deliver clean metal (free of slag and dross) at a
rate and velocity sufficient to completely fill the mould cavity before
freezing [2].
(v) Metal flow should be maintained in such a way that no gating or mould
10
(vi) The ideal, optimum gating system should avoid re-oxidation of metal in
(vii) The metal entry into the mould cavity should be properly controlled in
(ix) The gating system design should be economical and easy to implement
(x) The gating system must be economically practical; that is, it must not be
too expensive to mould and the quantity of metal used in the system
should be kept the minimum amount that will produce the desired
result [2].
Foundries should follow the practice of designing and testing their gating
systems on one or more pilot castings and then mounting the gates and runners
directly on the pattern equipment before a production run is started. The gating
system should be carefully designed so that the casting produced conforms to the
The parts that constitute a gating system are shown in Fig. 2.1 [25].
(ii) Sprue
(iv) Runner
(vi) Gates
The molten metal is not directly poured into the mould cavity
because it may cause erosion .Molten metal is carried in a ladle from the furnace
to some type of pouring basin on or in the top of the mould. Molten metal is
poured into a pouring basin which acts as a reservoir from which it moves
smoothly into the sprue. The main purpose of the pouring basin is to establish a
proper flow system as rapidly as possible. For metals such as Al and magnesium,
which react quickly when exposed to air, it is desirable to have a separate pouring
basin made of dry sand core or cat iron on top of the mould. Sometimes, a funnel
shaped opening is made at the top of the sprue in the cope itself, which serves as a
pouring basin.
12
( Downsprue,
Sprue 6a;
(Button,
Crossgote )
Runner Extension
13
(ii) Sprue
The vertical passage through the cope and connecting the pouring
The sprue size should satisfy certain conditions, for instance, the sprue
must be small enough for (a) the pourer to keep it full during the entire pouring
operation, and (b) the metal to enter the mould cavity at a velocity that avoids
spluttering and turbulence. At the same time, the sprue must be large enough for
(a) the mould cavity the fill completely without laps, seams or misruns, and (b) a
metal head to build up quickly enough to prevent mould gases from being
aspirated into the metal. Sprue sizes usually vary from 10mm square for work
This is a reservoir for metal at the bottom of the sprue to reduce the
momentum of the molten metal. The molten as it moves down the sprue gains in
velocity, some of which is lost in the sprue base well by which the mould erosion
is reduced. This molten metal then changes direction and flows into the runners in
(iv) Runner
In large casting, molten metal is usually carried from the sprue base
to several gates around the cavity through a passageway called the runner. When a
mould has more than one cavity, the common gate supplying metal to a number of
14
cavities is also called a runner, and the branches from the runner to the respective
mould cavities are referred to as in-gates. The runner may be positioned around
runner is generally preferred in the drag, it may sometimes be located in the cope,
depending on the shape of the casting. The runner should be stream lined to avoid
through each in-gate, the path of the runner is reduced in area after each
successive in gate by an amount equal to the in-gate area. Such `multiple in-
ingate.This extension is provided to trap the slag in the molten metal. The metal
initially, comes along with the slag floating at the top of the ladle and these flows
straight, going beyond the ingate and then trapped in the runner extension [21].
(vi) Gates
The gate is the passage that finally leads molten metal from the runner into
the mould cavity. The location and size of the gates are so arranged that the mould
can be filled in quickly with a minimum amount of cutting of the mould surfaces
by the flowing metal. The gates should be so placed that cracks do not develop
when the metal cools. The gate connections should be located where they can be
readily removed without damaging the castings. In-gates should not be placed too
near the end of the runner. If necessary, a well may be provided at the runner end.
IN
According to their position in the mould cavity, gates may be broadly
classified as
Molten metal is poured down the head or riser of the casting. Since
the metal falls directly into the mould cavity, the mould should be hard and strong
The advantage of top gating is that since all the metal enters the
casting at the top, the hottest metal remains in this region. As such, proper
temperature gradients are formed, and directional solidification towards the riser,
located at the top of the casting, can be achieved. The gates themselves may be
made to serve as risers. To prevent loose sand and drops from entering the mould
cavity and to allow the metal to fall in a small stream, a large size pouring basin
These gates enter the mould cavity along the parting line separating
the cope and drag portion of the mould. They may contain devices such as skim
bobs or relief sprues to collect dross or slag or relieve pouring pressure. This
function can be served by the use of pouring basin. Use of shrink bob serving the
dual function of slag or dross collector and metal reservoir to feed the casting as it
16
(c) Bottom Gates
The bottom gate enters the casting cavity at the bottom of the drag
half of the mould. For example, a well at the base of the sprue or a change in the
direction of flow of the metal may be incorporated to reduce flow rates in the
erosion and gas entrapment and to prevent splashing, which can result in cold
The metal continues to lose its heat as it rises in the mould cavity,
and by the time it reaches the riser, it becomes much cooler. As such, directional
solidification is difficult to achieve. It is difficult to place the riser near the gate
17
hJ IN
1d!
to) • 6')
(a) Open pours (e) Wedge gates
(b) Edge gate (f) Finger gate
(c) Pencil or Pop gate (g) Ring gate
(d) Gate with strainer core
Figure 2.2 Different types of top gate [18]
18
C
(al (b)
(c)
(d)
(a) Simple parting line gate (b) Gate with skim bob and choke (c) Gate with
strainer core (d) Gate with shrink bob (e) Branch gate (f) Parting line gate fed into
the riser
19
(a) C4
Worn sprue 1. Downgole (c)
/.'o/d cov/1y
(b) L Choke
r Sk%m bob
.j. "7
Draw In bol/omgc/e
' (e)
Choke
1)
P
(d) Miscellaneous Gates
Such gates are used for heavy and large castings. The molten metal
vertical steps. The size of ingates are normally increased from top to
bottom such that metal enters the mould cavity from the bottom most gate
and then progressively moves to the higher gates [21]. This ensures a
gradual filling of the mould without any mould erosion and produces a
The liquid metal that runs through the various channels in the mould obeys
the Bernoulli's theorem which states that the total energy head remains constant at
any section. The same stated in the equation form ignoring frictional losses is [21]
h+ p + V = Constant
Pg 2g
21
I
:rte •• •'~ ir:'~ •.:• :,~~~. ~: :~c
hi
t
a jfr
o ~ Sprue
LI 3 J D`
t3 l"gater
Ca) !C1 rdl
22
Bernoulli's theorem, which is based on the first law of thermodynamics,
The potential energy of the metal can be considered a maximum as the metal
enters the pouring cup or basin with no kinetic or pressure energies. This form of
energy is then rapidly changed to kinetic or velocity energy and pressure energy as
the metal passes through the mould system. Once flow is established and the
potential and frictional heads are virtually constant, the velocity is high when the
pressure is low, and vice versa. While metal is flowing, there is a constant loss of
energy in the form of fluid friction between the metal and mould wall. There is
also a heat loss which is not represented in Bernoulli's theorem, but which
system behavior, is the law of continuity which says that the volume of metal
flowing at any section in the mould is constant. The same in equation form can be
as follows.
Q=A1 V1 =A2V2
The law of continuity applies only to channels that are completely full; it
states that, since liquids are incompressible, flow rate Q must be the same at a
23
(c) Turbulence in the gating system
this is meant that the individual metal atoms do not flow in straight "streamline"
paths down the gating system, but travel from side to side as well as forward. At
sufficiently low metal velocities, true "streamline" flow can be achieved but these
velocities are so small that it is nearly always impractical to design a gating system
to obtain them.
It has been established that the flow of all fluids in ducts can be related by
p VD
Re =
µ
laminar, with the molecules of the liquid tending to move in straight lines without
turbulence. If a fluid flow system has a Reynolds number between 2000 and
20000, some mixing and turbulence will occur but a relatively undisturbed
boundary layer will be maintained on the surface of the stream. The flow in these
systems is nearly always turbulent. This type of turbulent flow, common in most
surface is not ruptured, thus avoiding air entrainment in the flowing stream. With
a Reynolds number of about 20000, flow will be severely turbulent. This will lead
to rupturing of the stream surface with the strong likelihood of air entrainment and
dross formation as the flowing metal reacts with gases [l].The degree of
24
turbulence encountered in well-designed gating systems does not appear to be
harmful to metal quality, although when it becomes excessive damage results from
(i) rupture of the liquid metal skin with consequent gas entrapment and (ii) mold
erosion with consequent sand or dirt entrapment [25]. In practice, the design of
gating systems does not involve the elimination of metal turbulence, but rather its
(d) Aspiration
sand moulds. Thus aspiration of gases from the mould into the metal is quite
When the pressure in the gating or mould cavity becomes less than the
atmospheric pressure the gases are aspirated into the metal stream. The gases lead
to the formation of oxides and dross, dissolve in the metal to precipitate later upon
V2
P= Pg° --+h--
Pg 2g
by maintaining pressures inside the mould and gating equal to or greater than the
such a way that they always run completely filled, thus eliminating turbulence and
25.
low pressure dead zones. Aspiration calculations are based on Bernoulli's
equation.
(e) Streamlining
Advantages of streamlining:
(f) Fluidity
As used in the foundry, the term "fluidity" means the capability of a molten
metal to fill a mould. The standard fluidity spiral is used to test the fluidity of a
given molten metal. Molten metal is poured through a channel of standard height,
and the measure of fluidity is the distance the metal will flow before the leading
edge freezes.
The most effective way to increase fluidity is to add superheat to the molten
metal. Care must be taken when increasing superheat [2], however, because too
much superheat can damage both the mould material and internal structure of the
metal.
All the metal that is used while pouring is not finally ending up as casting.
On completion of the casting process, the gating system used is removed from the
solidified casting and re-melted to be used again as raw material. Hence, the
casting yield is the proportion of the actual casting mass to the mass of metal
poured into the mould. This figure is always less than one, but higher the casting
consideration to maximizing the casting yield, at the design stage itself [21].
Casting yield depends to a great extend on the casting materials and the
complexity of the shape. Generally those materials which shrink heavily have
lower casting yields. Typical casting yield are presented in Table 2.1
27
Casting description Casting yield range
Steel
Simple shape 0.75 to 0.85
Cast iron
Heavy machinery parts 0.65 to 0.75
Small pieces 0.45 to 0.55
In designing a gating system contemplation is the first and perhaps the most
1. Maximum use of the mould volume for casting while leaving adequate
2. Position the parting plane of the casting to minimize the need for cores.
4. Attempt to put as much of the casting in the cope as possible to provide for
6. Use identical gating and risering for all identical casting in multiple
impression molds.
After these preliminary considerations the actual design of gating is done,
beginning with the calculation of pouring time and then the design of individual
components.
The time for complete filling of a mould termed as pouring time is a very
important criterion for design. Too long a pouring time requires a higher pouring
temperature and too less a pouring time means turbulent flow in the mould which
makes the casting defects prone. There is thus an optimum pouring time for any
The time required to fill the casting and riser cavities is the primary
concern, and the time to fill the gating system is incidental. However, as a
practical matter, measurement of the pouring time includes the time required to fill
sprue and runner system. The time to fill the gating system is estimated to be 10%
of the time of fills the castings and risers. The measured pouring time is the sum of
casting section thickness and casting size. The various relation used are not
calculating the optimum pouring time in addition to the mass of the casting itself.
Normally while considering the mass of the casting, it may not be necessary to
29
consider the mass of the gating system because the gating system is completely
filled before metals starts entering the mould cavity. However, if the gating
systems are in comparable size to the actual casting, it may be desirable to include
The following are some standard methods to calculate the pouring time for
fW
t=k 1.41+ T sec.
C 14.59
T
kIl.236+
t
16.65
rw sec.
sec.
30
(v) Copper alloy castings [19]
t = K 2 ;/W sec.
(vi) Intricately shaped thin walled castings of mass unto 450 kg.
sec.
Thickness, T (mm) K3
1.5 to 2.5 1.62
2.5 to 3.5 1.68
3.5 to 8.0 1.85
8.O to 15.0 2.80
Table 2.2 Value of constant K3 for different values of average thickness [21 ]
31
Thickness,T(mm) K4
up to 10 1.00
10 to 20 1.35
20 to 40 1.50
Above 40 1.70
Table 2.3 Value of constant K4 for different values of average thickness [21]
(viii) Light Metal Alloys: Unlike steel, light metals like Al and magnesium
and their alloys are poured at a slow rate. This is necessary to avoid
Pouring rate = W
~86 +1.09 T
F
liquid flowing into the mould by settling first into it. Its shape is like that of a
funnel cup which forms the top portion of the sprue. A basin must be large enough
to take care of the small variation in pouring rate so that there is no overflow or
emptying of the basin. In order that vortex is not formed during pouring, it is
necessary that the pouring basin be kept full. Further provision should be made in
the pouring basin so that constant conditions of flow are established. This can be
achieved by using delay screen or a strainer core. The metal should be poured
32
steadily in the pouring basin keeping the lip of the ladle as close as possible. A
large basin will enable the slag and dross to float over the metal surface and
prevent them from entering the sprue. A few design of pouring cup are shown in
Fig. 2.7.
(2) Providing a dam to establish uniform flow conditions, shown in Fig. 2.8
As the metal falls freely down the sprue it accelerates under the influence of
gravity and its velocity is proportional to the square root of metal head, hence
ideally.
Y= 2gh
Due to increase in velocity the metal stream contracts inwards and is pulled
33
So the sprue should be tapered down to take into account the gain in
velocity of the metal as it flows down reducing the above effects. The exact
AtV, = AV
A,=A-
h
h
so A, =ALii
The square root suggests that the profile of the sprue should be parabolic if exactly
done as per the above equation. But making a parabolic sprue is too inconvenient
in practice and therefore a straight taper is preferable. It has been found in practice
that a straight tapered sprue is able to effectively reduce the air aspiration as well
In order to arrive at the dimensions of the sprue at the top and the
subsequent taper one has to consider the head of metal in the pouring basin as
shown in Fig. 2.10. Metal at the entry of the sprue would be moving with a
At = A`
ht h, +H
34
\;_1:1
(Li I•.. il:
Figure 2.7 Various type of pouring cup basin [25]
JP4'e ,O/U7
(a)
/ V
00"
0
1. /cw/d
etc/
Al
35
PERSISTS
THIS CONDITION
SOON CLEARS UP
Figure 2.9 Effect of sprue design on metal turbulence and aspiration [25]
•I - - - - .1-fr,-
- I- - . - -
- - S -
-
J1
- •
Straight •'. t
I •••
. .
I
I. •: •'
I S
ht
Toper
ill
Parabolic
Taper
36
The effective sprue height (H), of a mould depends on the casting
dimensions and type of gating used. The effective sprue height can be calculated
The values of h, P and C are shown in Fig. 2.11 [26] for the various types
of gating. Table 2.4 shows the theoretical values of area ratios of top and choke
portions of the sprue based on sprue height and metal head in the pouring basin.
Table 2.4. Theoretical ratios of the sprue top and choke areas based on pouring
37
2.4.4 Design of Choke Area
the main control area which meters the metal flow into the mould cavity so that
the mould is completely filled with in the calculated pouring time. This controlled
area is called choke area. Normally the choke area happens to be at the bottom of
the sprue. The main advantage in having sprue bottom as the choke area is that
A_ W
ptC 2gH
Table 2.5 Values of Efficiency coefficient for various type of gating system [26]
The provision of a sprue base well at the bottom of the sprue helps in
reducing the velocity of the incoming metal and also the mould erosion.
Reasonable proportions for a sprue base well are shown in Fig 2.12 a general
t:
guide line could be that the sprue base well area should be five times that of the
sprue choke area and the well depth should be approximately equal that of the
runner [21]. For a narrow and deep runner, the well diameter should be 2.5 times
the width of the runner in a two runner system and twice its width in a one runner
system.
Fleming and Taylor recommended that a sprue base or sump should be used
to absorb the vertical metal velocity. This sump should have a diameter 1.5 times
the runner bar width and a depth 1.5 times that the runner bar. As the sump will be
Ingate is a channel through which the metal leaves the runner to enter the
mould cavity [9]. The ingate can be considered as a weir with no reduction in
cross section of the stream at the gate. Then the rate of flow of molten metal
through the gates depends on the free height of the metal in the runner and the gate
area and the velocity with which metal is flowing in the runner. The free height, h
~2 V2
h=1.6 3 +-mm
gG"„Z 2 g
Having obtained the head of the metal, the height of the gate h is give by
h1 = h — 5 mm.
Gates higher than this will not fill completely the gates and those lower
than this will increase the velocities of the stream entering into it.
39
(a) TOP GATING 7
C
ErFCCTfVE HEAO. Hal
.L
r
(b) BOTTOM GATIN4
i C
_1.
ErrECTIVE HEAD, H•h—
_4. C
(c) PARTING ZINC GAT.NC
H%[rrCCTIV[ MCAO~
c
2C
11'x~r'~'I)'ifo/fr+A
40
2.4.7 Design of Runner
metal from the sprue to the ingates. The number of runner bars required is largely
determined by the size and shape of the casting. The performance of a runner is
dependent upon.
(1) Abrupt changes in section and sharp corners which create turbulence
(2) Proper position of sprue and ingates in the runner must be done
(3) The runner should be kept full of liquid metal to avoid aspiration
(4) The flow must be controlled in the runner to that a quick mould
filling is possible
several gates, between gates and runners and between the runners and the sprue.
41
RA = total cross-sectional area of runner
The gating ratio provides the desired distribution of metal and favorable
After deciding the gating ratio and runner area its shape should be chosen.
Henzel [18] has shown that a runner of circular cross-section suffers least heat loss
and has the smoothest flow. However, as this is difficult to mould accurately, the
runner should have a width: height, ratio of 1:1.5 and that the ingates should be
positioned at its base so that the runner is situated in the cope. This practice traps
provide a runner extension [20]. The runner extension is a typical feature of the
runner design. It extends beyond the farthest ingate as a blind end. Although the
length the runner is governed by the position of the ingates and the sprue, the
runner should extend part the last ingate, so that the first flush of metal, which is
liable to contain oxides and loose sand is retained in the extension. There appear to
be no rules for determining the optimum length of the extension and it is often
Webster [18] suggested that as the metal flows through the runner its
area of the runner away from sprue, in accordance with the law of continuity.
42
But Grube and Eastwood [18] suggested that the gates nearer the sprue will
have less metal flowing through them because of higher velocities and lower
pressures, while the farthest gate will have more inflow due to higher pressure
there. This will lead to an uneven distribution of metal. Thus it was suggested that
the runner beyond each gate should also be reduced in cross-section to balance the
distribution and flow of metal. However, the final choice of the runner cross-
sectional area depends on the gating ratio employed and fulfillment of flow
characteristics.
If the primary chokes of the feeding system in the sprue, the balance of the
deviations that can be encountered in pouring practices for other metals are
Thus it is seen in Fig. 2.13 and Fig. 2.14 that the gating system can be two
A non-pressurized gating system having choke at the bottom of the sprue base,
having total runner area and ingate areas higher than the sprue area. In this system
there is no pressure existing in the metal flow system and thus it helps to reduce
turbulence. When the metal is to enter the mould cavity through multiple ingates,
the cross section of the runner should accordingly be reduced at each of a runner
break-up to allow for equal distribution of metal through all the ingates.
43
In the case of a pressurized gating system normally the ingate area is the
smallest, thus maintaining a back pressure throughout the gating system. Because
of this back pressure in the gating system, the metal is more turbulent and
generally flows full and thereby, can minimize the air aspiration even when a
The gating system cab be very widely from one leading to a non-pressurized or
`reverse choke' system, such as 1:3:3, to one where the choke and pressure are at
a maximum at the ingate such as 1:0.75:0.5. If more than one ingate is used, the
ratios pertain to the total area of the all the ingates [11]. In other words, in
changing from one ingate to two while maintaining the same ingate ratio, the area
of the two ingates should equal that of the single ingate system.
The difference between these two systems is in the choice of the location of
flow-controlling constriction, or choke that will determine the ultimate flow rate
for the gating system. This decision involves the determination of a desired gating
ratio, that is, the relative cross-section areas of the sprue, runner, and gates. This
ratio, numerically expressed in the order sprue: runner: gate, defines whether a
Common non-pressurized gating ratios are 1:2:2, 1:2:4, and 1:4:4. A typical
44
Metal and Alloy Gating Ratio
Aluminum 1: 2 : 1
1:1.2:2
1:2:4
1:3:3
1:4:4
1:6:6
Aluminum bronze 1: 2.88 : 4.8
Brass 1 : 1 : 1
1:1:3
1.6:1.6:1
Copper 2:8:1
3:9:1
Ductile iron 1.15: 1.1 : 1
1.25:1.13:1
1.33 : 2.67: 1
Grey cast iron 1: 1.3 : 1.1
1:4:4
1.4:1.2:1
2:1.5:1
2:1.8:1
2:3.1
4 3 1
Magnesium 1:2:2
1:4:4
Malleable iron 1: 2 : 9.5
1.5 : 1: 2.5
2:1:4.9
Steels 1 : 1 : 7
1:2:1
1:2:1.5
1:2:2
1:3:3
1.6:1.3:1
Table 2.6 Some gating ratio used in practice for different metals and alloys [211
45
IJI
(Gating Ratio- 1: 3: 3)
46
CHAPTER 3
RISER DESIGN
Shrink: The difference in volume between liquid metal and solid metal or
shrinkage alloys require that extra metal flow into the casting during
metal to solidify.
Feed metal: The volume of liquid metal that passes from the risers to the
Open Riser: A riser which cuts through the cope surface of a mould.
Blind Riser: A hidden riser, not visible at the top surface of the cope. Blind
risers are generally more efficient and less expensive than open risers, but
47
Riser_
Riser
height nr
:. ser Riser neck
Riser pad
height
Riser ,/•
Pouring distance I
Cup : •Side
:rlserr
Casting j
Downsprue
Runner
Riser base
Riser neckJ1 Riser vad
48
Riser Height: The distance from the top of the riser when liquid to top of
the riser neck. The height of riser when solid may be several inches less
Riser Neck: The connecting passage between the riser and casting.
Usually, only the height and width, or diameters of the riser neck are
Riser Pad: An enlargement of riser neck where it joins the casting. The
purpose of the pad is to prevent the riser from "breaking in" when it is
Riser Distance: The length of the riser neck. The term is applied to side
risers only.
Riser Gating: Practice of running metal for the casting through the riser to
reservoirs of feed metal in addition to the desired casting shape so that undesirable
shrinkage cavities in the casting are eliminated or moved to locations where they
are acceptable for the intended application of the casting. When metals solidify
and cool to form a casting, they generally undergo three distinct stages of volume
(i) Liquid shrinkage: The liquid metal loses volume as it gives up superheat
49
(ii) Solidification shrinkage: The metal freezes, changing from a liquid to a
higher-density solid. For pure metals, this contraction will occur at a single
temperature, but for alloys it will take place over some temperature range or
freezing interval.
(iii) Solid shrinkage: The solid casting cools from its solidification temperature
to room temperature. The volumetric shrinkage of various metals and alloys are
Nickel 6.10
Monel 6.30
Aluminum 6.60
Copper 4.92
Magnesium 4.20
Zinc 6.50
Table 3.1 Volumetric shrinkage for different metals and alloys [21 ]
* ~I 50
\II T. ROOR~'84
Liquid
Solid
(a) (b)
Void
C~ O
51
The last of these, solid shrinkage (also called patternmaker's shrinkage), is
accommodated by making the pattern (and therefore the mould cavity) somewhat
longer than the desired dimensions of the final casting. Liquid shrinkage and
defects will vary with different alloys; for example, internal shrinkage may be
more dispersed, or alloys with strong skin-forming behavior may not exhibit
will be added to accommodate the liquid shrinkage and to supply liquid feed metal
to compensate for the solidification shrinkage within the casting. Therefore, the
shrinkage in the riser/casting system is concentrated in the riser, which will then
produced on cooling in the central region of the mass of the molten metal poured
into the cavity of the mould. The reason for the formation of the void in the
casting is that the liquid metal in the centre which solidifies in the end is not fed
during the solidification, hence the liquid shrinkage occurred end up as a void.
Riser is a vertical channel in the mould generally located at the top or side of the
mould cavity. When molten is also filled up in this vertical channel it provides a
column of reservoir of molten metal connected with the casting. When molten
52
metal in the casting cools, it shrinks in volume. To compensate for this shrinkage,
the molten metal is supplied from the riser column. Thus, riser is an ingenious
device which if properly designed will force the shrinkage voids or cavities to shift
from within the casting to the riser. Riser is thus an extraneous portion cast as an
integral but distinct portion of the casting like gating system. After the casting has
solidified, all extraneous parts including riser, are cut off from it leaving behind
This leads to the following two basic requirements of the riser design:
(i) The molten metal in the riser must solidify only after the molten metal in
the main casting has solidified. This will ensure that the riser has molten metal
ready to supply to the location in the casting where shrinkage void could have
(ii) The riser volume should be as small as possible but sufficient enough to
supply the molten metal needed for compensating the shrinkage in the volume of
Most of these risers use much more material than is actually required.
Certain attempts for determining the optimal designs of risers have recently been
made. However, these investigations are not exhaustive and there is a need for
further investigations in this direction. Although many method of riser design have
53
3.2.1 Chvorinov's method
The earliest and most widely known quantitative risering analysis is that of
Chvorinov. Chvorinov showed that the time for complete solidification of a cast
t=K V z
(SA)
longer than the casting it is to feed. With this in mind, chvorinovs rules serves as a
proportional to volume and the rate of heat dissipation depends on the surface
V V
SA Ricer SA casting
It may be argued that the sphere has highest volume-to-area ratio and hence
that it should be used as a riser, but spheres are usually difficult to mould however,
and would present feeding problems as well, since the last metal to freeze would
be near he centre of the sphere, where it could not be used to feed a casting. The
next best shape of riser is the cylindrical type which is most commonly used for
54
3.2.2 Caine's Method
According to caine, if the casting solidifies very rapidly, the feeder volume
need be only equal to the solidification shrinkage of the casting. On the other
hand, if the feeder and casting solidify at the same rate, the feeder must be
infinitely large. This signifies that hyperbolic relationship exists between relative
Caine uses the ratio (surface area/ volume), which is inversely related to the
function of the surface of the casting, while the heat content is a function of
volume. It is assumed intuitively that the linear relation (SAN) determines caning
rate and hence is inversely related to freezing time. While the chvorinov
relationship has a firmer theoretical basis, the simpler caine ratio apparently falls
within the limit of error, considering the many other factors involved.
a
X= +c
y—b
(SAl V)ec,,t,n g
x=
(SA IV)ruer
55
The x-axis represents the freezing ratio and y-axis represents the .volume ratio of
riser to casting,
= Riser volume
Y
Casting volume
where a, b and c are constants whose values for different materials as shown
in table 3.2.
Material a b c
Steel 0.10 0.03 1.00
Aluminum 0.10 0.06 1.08
Cast iron, Brass 0.04 0.017 1.00
Grey cast iron 0.33 0.030 1.00
Aluminum bronze 0.24 0.017 1.00
Silicon bronze 0.24 0.017 1.00
The following equations are used for calculating the risering requirements
for Aluminum alloys LM4 (Cu 2-4%: Si 4- 6%) and LM 11(Cu 4-5%) [19].
33.45
LM4: X =
23.98—y
X = 17.11
LM 11:
12.78 — y
LM4:X= 32.09
23.98—y
56
1.4
U 1.2
O
1.0
0 Sound casting
ps 0.8
w 0.6
0.4
Shrinkages
0.2
0.0
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
Freezing ratio
57
LM 11: X= 16.75
12.58 — y
With the help of a graph which is as shown in Fig 3.3, the values of X and
Y may be potted. If they meet above the soundness curve, the value the selected
riser size will be satisfactory. If the meeting point is below the curve, the riser will
Another method for finding the optimum riser size is the `modulus
established that if the modulus of the riser exceeds the modulus of the casting by a
volume) as defined earlier. In the other word, modulus is defined as the ratio of
Most authorities and researchers in risering agree that the minimum height
of a riser should be no less than one half of its diameter and the maximum height
should be no more than one and a half times its diameter [3]. In steel castings, it is
Mr = 0.2Dr
Since, Mr = 1.2 M,
Dr = 6 M,
58
Thus in this method, the calculation of the riser size is simplified to the
calculation of the modulus of the casting itself and no trail and error solution as is
given in the previous case. Though, this takes into account the cooling effect of the
riser, it does not consider exactly the amount of feeding metal required to
compensate for the shrinkage of the casting. If allowance is made for the volume
of metal to be fed to counteract the contraction of the casting, the equation would
change to
The above is valid when the height to diameter ratio of the riser is one.
When the third term in the equation . relating to feed volume is neglected, the
previous simplified equation would be arrived at. With chunky castings, e.g.,
cubes, the volume component may be negligible, but for those rangy castings,
approximation [21].
59
The freezing time of risers and castings are proportional to their respective
moduli, and if the modulus of the riser, Mr, is sufficiently greater than the modulus
of the casting, M,, good feeding will be obtained. In steel, if Mr = 1.2 Mc, feeding
will be satisfactory. For other skin forming alloys, including many aluminum and
copper-base alloys, the Mr /Mc ratio of 1.2 : 1 is appropriate. With grey and ductile
irons, depending on the carbon equivalent, the required M r/Mc ratio can range from
0.8:1 to 1.2 : 1 because the riser may not be required to supply feed metal
requirements and the feed metal requirements. These dimensions vary with the
fraction of the riser available for feeding and the minimum size which meets both
the solidification time and feed metal requirements is selected as the optimal
size [6].
From Chvorinov's rule, the solidification time of riser must be greater than
tr ~ tc (1)
60
Shape Modulus, M~
Disc d 5 5t 0.51
a
axb
Long bar
2(a + b)
b
Cube T D
6
D
Cylinder U"
6
1
D
Sphere D
6
DH
Cylinder
r
2(D+2H)
Annulus
1J H
rH
2(r + H)
cross-sectional area
Any bar with no cooling ends
perimeter of cross section
11
There is a transfer of metal from the riser to the casting as the casting
directionally solidifies to the riser. Under the conditions of complete feeding, the
amount of metal transferred to the casting would be (3Vc The amount of metal
available for feeding is aVr. The connecting area of the riser must be subtracted
from the initial surface area of the casting to properly evaluate the casting modulus.
(i) nr = n, ; the solidification exponents of the riser and casting are equal.
(ii) Br = Bc ; the solidification constants for the riser and casting are equal.
(iii) a >_ f3; the riser fraction available for feeding must be greater than the
volumetric contraction.
Vr = 7cDr2Hr / 4 (6)
Ar = 7tDr2 /4 (8)
Using the equations (6) (7) and (8), in equation (5), reducing the terms, and
rewriting the equation in the standard geometric programming form, equation (5)
becomes:
62
nD,' 1434 +4D -'V.(1+ (3)/((1-a)S4,) = H -'V~.(1+ (3)/((1-a)SA,) < 1 (9)
The solution which minimizes the riser volume produces the following
Vrt = nDr2Hr / 4 (12)
When the feed metal is critical, the feed metal from the riser must be
sufficient to feed the casting. This can be expressed by stating that the feed metal
available for the casting must be equal to the feed metal required by the casting i.e.
V@-l) = VCP
Vrf = Vca/(a — a) (13)
The value of a must be greater than (i and less than the maximum value
After several years of successful use of the caine risering calculation, the
NRL group worked out a new and simplified procedure which has the advantage
of eliminating trial and error calculations. The method is based on the observation
that the ratio (riser volume/ casting volume) must be greater for chunky castings
because of their relatively low surface/ volume ratios. This requirement is also a
63
characteristic of the Caine relationship [9]. This method defines a shape factor to
replace the freezing ratio. The shape factor is defined by adding the length and
width of a casting section and dividing this sum by the section thickness i.e.,
too complicated and therefore simplification would be desirable. The length, width
and thickness are computed by using the maximum dimensions of the parent
Then, the ratio of the riser volume to casting volume can be obtained from
graph shown in Fig 3.5. This shows when sound castings would be obtained for
C20 to C50 steels. Having obtained the riser volume, the reference may be made
to Fig 3.6 to obtain riser diameter and height for the obtained riser volume. It has
been proven empirically that for side risers the height to diameter ratio is 1 and for
For circular plates, the length and width are same as that of the diameter.
But for cylinders, the width and thickness are same as the diameter for calculating
the shape factor. But for calculating the riser volume, the actual casting volume is
to be used.
64
1.0
A N RL Experiment point
• Calculated points (Caine method)
All risers have. HID in range 0.5 to 1
1
0.8
A rD
i .
0 0.6
0.4
0
0.3
0.2
0.1
0.0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36
L+W
Shape factor T
65
The values obtained by NRL method are generally conservative in nature
and corresponds to Caine's method for most of the simple geometries. The NRL
method, however, has been further extended to include calculations for complex
casting shapes such as shown in Fig 3.7. When ribs or other appendages are thin,
they do not appreciably increase the freezing time of the main portion of the
casting, and therefore only a small increase of liquid metal is needed in the riser to
allow for the appendages. As the appendages become heavier, the riser must be
increased considerably to ensure that the freezing time of riser is sufficiently larger
than that of the casting. Very thin fins can be used to reduce the cooling time;
however the effect of such an arrangement is difficult to calculate and is not taken
added, due to addition of branches (parasitic volumes) can be estimated from the
Fig 3.8 based on the thickness variation. Because of the greater surface area, a bar
Hollow cylindrical shapes such as bushings also present a special case. The
heat flow from the center core is restricted, and he casting as a whole has a lower
cooling rate than a plate of the same cross-section. The simplest approach is to
consider the shape as a plate, but to correct for an effective plate thickness (as
1111
6
IIIUU1l■
5
0-
0
Ii
50 100 150 200 250 300 350 400 450 Wuu
Riser volume. cmI
12
11
10
a 7
I
4 500 700 900 1100 1300 1500 1700 19(X) 2100 2300
Riser volume, cm.
67
22
20
18
nii
to
h SW4SI
I5(M) 2(MMI 25(() 30(X) 40(M) 50(0 6000 6500
Riser volume, cm3
Figure 3.6 Selection chart for riser dimensions based on NRL method [21]
u 60
a
0
40 4UUI
rim
20
0
0 0,2 0.4 0.6 0.8 1.() 1.2 1.4 1.6 1.8 2.0
Thickness parasitic/thickness parent
Ell
Let T be the true wall thickness and T, the effective plate thickness, then
T= kT
L+W
Shape factor =
kT
The solidification mode of the casting affects its structure and soundness. It
occurs by the nucleation of very small grains or crystals which grow under thermal
and crystallographic conditions till the complete melt solidifies. The crystal
generation.
Depending on the location, the riser is described as a top riser or side riser
and may be either an open riser or blind one. Since risers are designed to stay
liquid while the casting solidifies, the riser shape and size and the feeding distance
are the important aspects in its design. The riser should be placed within effective
feeding distances of the casting sections requiring liquid metal. Feeding distance
solidification. The Feeding distance of a riser is the length upto which the riser can
supply liquid metal. Thus, the riser feeding distance should be greater than the
When a long bar or plate is cast without a riser, it found that a certain
length from each end of the bar or plate is sound. This result from the directional
solidification that develops at the ends because of the greater heat extraction from
those points compared with others. This effect occurs despite the absence of a
riser. Similarly, if a long bar or plate is cast horizontally with one adequate riser at
the centre, it will be found that, for a certain distance from the riser, the casting
will be sound because of the feeding action of the riser, whereas beyond this
points, some form of shrinkage will be evident. These two effects can be referred
All of the preceding calculations are based upon the assumption that the
riser or risers are placed within the effective feeding distances of the casting
sections requiring liquid metal. Each combination of alloy and mould material
determine experimentally the effecting feeding distance for each combination until
While calculating the risering dimensions, it was assumed that the riser
would be able to feed, whatever the length the casting may be. If the casting is
long, not the entire casting would be sound because the riser would not be able to
feed the entire length of the casting. In the determination of feeding distance, it is
assumed, for simplicity, that any casting can be divided into plate, bar, cubical and
71
spherical sections. The cubical or spherical sections offer no problem of feeding
distance because the riser can be placed near the location to be fed. Hence, if we
can develop feeding distance data for bar and plate sections, we shall be able to
Naval research laboratory after extensive tests on steel casting has arrived
at a number of rules for these feeding distances. Their results are presented in Fig
3.9 for bars and in Fig 3.10 for plates. Similarly for those shapes with varying
It can be noted from the Fig 3.9, 3.10 and 3.11, that the effective feeding
distance of a riser is more in a plate than in a bar because in the former the
solidification progresses only from two sides whereas in the latter, it progresses
72
Max, distance
Length greater than max. distance
4
~ ST 4.5T 2T
r''1 2.ST
T _
_I14 j4 Sound lIT
27 2.5T Variable
Riser Edge Centerline shrinkage
contribution contribution
Max. distance
4T
2T 2T
IT .
Length greater than mu. distance
P~-2"
T V
~T
73
n - i rr Ir % .
I.5 trim
It
74
CHAPTER 4
SOFTWARE DETAILS
stored in the computer memory and can be executed at any time. The high
programming under an operating system called UNIX, which itself was later
BASIC, PL/I, and PASCAL. Where it differs is that C permits very close
software industry is adopting the language to great advantage. One reason for
transferred easily one comport to another with minimal changes or none at all.
75
desirable language in the highly competitive software industry. These features
interlinking with the help of software program. Computer offers the choice of
various alternatives like type of pouring basin, sprue, runners, choke location
for gating system and for riser designs offers the type of riser and dimensions
of riser.
parameters concerned in a very short time. The design engineer can select and
modify for the final design in view of the economic and functional
requirements.
First, all the system design parameters must be enlisted. This includes
all information about casting and factors affecting it. Then, a software program
should be written, which should be interactive and user friendly. The basic data
remains same for a particular casting must be input at the beginning of the
program. The other input data which are to be varied during the running of the
program to obtain the best design, should be input to the computer as and when
required in the program. These variable data include the choke area, metal
head, gating ratio and the ratios between the dimensions of the various
components of gating system. Finally, when the dimensions of the riser and
76
gating system have been calculated, the program should be capable of
accurately finding the proper size of rigging systems for a given casting.
GATING DESIGN
The steps listed below describe the approach used to design a riser and
gating system.
1. Information about the casting is the starting point. The casting volume is
required:
requirement then compare which one is greater after that decide the riser
volume.
5. Determine the pouring time for the weight of casting and riser.
6. Determine the choke cross sectional area required to deliver the liquid in the
time desired.
al) Select the geometry and calculate the dimensions of the gate(s).
a2) Calculate the sprue cross sectional area by gating ratio and the
a3) Calculate the total runner cross sectional area by gating ratio.
77
a4) select the geometry and calculate the dimensions of the runner(s).
b 1) Select the geometry and calculate the dimensions of the choke and
sprue.
b2) calculate the total runner cross sectional area by gating ratio.
b3) Select the geometry and calculate the dimensions of the runner(s).
b4) Calculate the total gate cross sectional area by gating ratio.
b5) select the geometry and calculate the dimensions of the gate(s).
The flow chat shown below has been designed specially for Aluminum
78
START
Input Parameters
Type of metal, Casting
dimensions, Gating ratio,
avg. casting thickness etc.
D=6
V ~ )(1+ p
(SA, 1-a
H V
SA,-3n 4
D
2 WN
iJJ
Vr =V
/ Hr n of Riser
Dimensins
79
A
Top Bottom
T e of Sprue height
Sprue height
Gating?
(H)=h
Parting Line
Choke area(A)
W
Cpt (2gH)
Pressurized Unpressurized
Gating ratio Types of Gatingg ratio
(SA:RA:GA) gating (SA:RA:GA)
system
R Gate area(Ag)=A*GA
Gate area(Ag)=A*GA S
Choke Sprue bottom area (A,)
Sprue bottom area (At)
location =SA*A
=A
Runner a~ea(Ar)=RA*A Runner area(Ar)=RA*A
Gate area(Ag)=A
Sprue bottom area (Ac)
=A*SA
Runner area(Ar)=RA*A
81
C
Gate area
Gate thickness
Gate width
Gate height
Runner area
Runner thickness
Runner width
Gating system
dimension
Reyonlds number
Re =pVd/µ
>20,000
Is Re ?
< 20,000
E D
82
83
Flow chart for the riser and gating system design
CHAPTER 5
On the basis of the flow chart, a program has been developed for the
design of riser and gating system. This program was developed using C
programming language. When the program is run, its output can be obtained.
This output predicts the desired characteristics and dimensions of the rigging
The following are the some of results obtained by running the program
with different inputs. Some of the inputs, which are kept constant for all the
cases and some of the input varies during the running program like dimensions
ratio etc.
85
RESULTS:
CASE I
Riser design
Riser volume=1097074.500000 cu.mm
Riser height=111.784660 mm
Riser diameter=111.784660 mm
Runner design
Individual runner area=608.701965 sq.mm
Runner thickness=17.445658.mm
Runner width=34.891315 mm
Gate design
Individual gate area=608.701965 sq.mm
Gate thickness=17.445658 mm
Gate width=34.891315 mm
Gate height=34.460033 mm
Turbulence check
Reynolds number at choke location=17081.255859
Riser design
Riser volume=702127.687500 cu.mm
Riser height=96.333099 mm
Riser diameter=96.333099 mm
Sprue design:
Sprue bottom area=151.812653 sq.mm
Sprue top area=303.625305 sq.mm
Sprue bottom diameter=13.903023 mm
Sprue top diameter=19.661842 mm
88
Sprue well design
Sprue well area=759.063232 sq.mm
Sprue well diameter=31.088102 mm
Sprue well depth'= 15.090361 mm
Runner design
Individual runner area=455.437958 sq.mm
Runner thickness=15.090361 mm
Runner width=30.180721 mm
Gate design
Individual gate area=455.437958 sq.mm
Gate thickness=15.090361 mm
Gate width=30.180721 mm
Gate height=45.261703 mm
Turbulence check
Reynolds number at choke location=17060.878906
89
CASE III
Riser design:
Riser volume=2434068.750000 cu.mm
Riser height=126.109192 mm
Riser diameter=156.764709 mm
Sprue design:
Sprue bottom area=190.566071 sq.mm
Sprue top area=381.132141 sq.mm
Sprue bottom diameter=15.576793 mm
Sprue top diameter=22.028912 mm
Runner design
Individual runner area=381.132141 sq.mm
Runner thickness=13.804567 mm
Runner width=27.609135 mm
Gate design
Individual gate area=762.264282 sq.mm
Gate thickness=19.522606 mm
Gate width=39.045212 mm
Gate height=67.119392 mm
Turbulence check
Reynolds number at choke location=19114.818359
91
CASE IV
Input parameters .
Type of metal = aluminum
Shape of the casting = cuboid
Average casting thickness of the casting = 50.000000
Length of casting = 350.000000
Width of casting = 200.000000
Thickness of casting = 50.000000
Type of riser = cylindrical
Type of gating = bottom
Type of gating system
Non-pressurized gating system
Riser design:
Riser volume=2457447.000000 cu.mm
Riser height=146.262009 mm
Riser diameter=146.262009 mm
Sprue design:
Sprue bottom area=249.092743 sq.mm
Sprue top area=498.185486 sq.mm
92
Sprue bottom diameter=17.808846 mm
Sprue top diameter=25.185513 mm
Runner design
Individual runner area=747.278198 sq.mm
Runner thickness=19.329746 mm
Runner width=38.659492 mm
Gate design
Individual gate area=747.278198 sq.mm
Gate thickness=19.329746 mm
Gate width=38.659492 mm
Gate height=45.894535 mm
Turbulence check
Reynolds number at choke location=19981.843750
93
CASE V
Input parameters
Type of metal = aluminum
Shape of the casting = cuboid
Average casting thickness of the casting = 50.000000
Length of casting = 300.000000
Width of casting = 250.000000
Thickness of casting = 50.000000
Type of riser = cylindrical
Type of gating = bottom
Type of gating system
Non-pressurized gating system
Riser design:
Riser volume=2632978.750000 cu.mm
Riser height=149.664673 mm
Riser diameter=149.664673 mm
Sprue design:
Sprue bottom area=268.304718 sq.mm
Sprue top area=536.609436 sq.mm
94
Sprue bottom diameter=18.482870 mm
Sprue top diameter=26.138725 mm
Runner design
Individual runner area=321.965668 sq.mm
Runner thickness=12.687901 mm
Runner width=25.375803 mm
Gate design
Individual gate area=536.609436 sq.mm
Gate thickness=16.380011 mm
Gate width=32.760021 mm
Gate height=122.816528 mm
Turbulence check
Reynolds number at choke location=19928.912109
95
DISCUSSION:
The following discussion gives the clear idea of the riser and gating
system design.
The riser and casting were both assumed to follows Chvorinov's rule for
solidification and the feed metal supplied by the riser. This method involves
the calculation of the riser volume based upon the sum of two components, one
representing the effect of solidification time and the other representing the
determining the optimal riser size. The maximum value of the fraction of riser
available for feeding is 0.16 in conventional method of casting and more than
however, the casting yield is considerably reduced. This clearly indicates that
the riser volume is quite high and this high volume is forced by the modulus
The gating system is designed for cuboid casting shape and its
dimensions are the input of the program. Pouring time is calculated for bottom
type of gating system, which depends upon thickness of the casting, casting
and riser weight. The total pouring time consist casting filling time, riser
filling time and gating system filling time, which is 10% of the casting, and
96
riser filling time. With the help of this pouring time, first calculate the choke
area then decided the location of choke, which depends on gating ratios, after
From the results, we can say as the pouring time increase, choke area
starts decrease. In the gating design, it probably reflects the wide range of
casting sizes made in this alloys as there is a tend to pour large casting at
higher rates. This covered different casting designs, gating ratios, gating
system etc. timing the pour and weighing the complete casting including sprue,
runner, gate, riser etc calculated the pouring rates. The poring rate is a function
Reducing the mold fill time allows more for correct thermal gradients
argued that high pouring rates lead to turbulence, which entraps gas and dross
inclusions, and causes more mold erosions. However, these defects are a
pressurized), the maximum achievable pouring rate is that which just keeps the
pouring basin and sprue full at all times metal spilled on the top of the mold or
on the floor obviously serves no useful purpose. The minimum pouring is one
essential to fill the sprue as quickly as possible, to keep it full throughout the
pouring the molten metal, and to use a tapered sprue of the correct proportions.
This prevents air aspiration and with an adequate pouring basin, minimizes the
entry if slag and dross from the ladle. With the sprue full, all molds are poured
at the maximum rate. This ensures reproducibility as the design of the system,
accelerates under the influence of gravity in the spure. We can see from this
mathematical expression.
V= 2gH
Although in the first place flow rate is a function of the velocity and
mold passage sizes, it is affected by other factors. These include friction losses,
which increase as the mold passage size and its volume to surface area ratio
decrease.
effects and indeed the flow rates measured were only about 60% of those
top diameter of the sprue and the height of metal in the pouring basin control
the delivery rate of metal. If a pouring basin is not used, then the height of the
gating ratios are used in this result. It is noted that that the velocity in gating
systems are far higher than those in the casting cavity so that most of the
problems ascribed to turbulence are probably caused before the metal level in
the casting rises above the top of the gates. With bottom gating, this occurs
quickly, the high gate inlet velocity is hydraulically damped, the exposed
may persist for some time. With top gating, where a substantial volume of the
casting is in the drag, the high rate velocities projected into the casting cavity
without damping until a late stage of the pour. Indeed, there is every possibility
of further acceleration under gravity as the gate streams either free falls or
flows over a steely sloping mold surface. This emphasizes the need for tapered
spure, non-pressurized systems, high gating ratios and bottom pouring for
dross prone alloys. It should also be noted that higher gating ratios are needed
with large castings using long sprue if initial turbulence and consequent dross
inclusions are to be avoided. It was shown that for the same size sprue, a non-
pressurized system should deliver more weight of metal per unit time than a
pressurized system.
dependent primarily on the height of the sprue. Metal velocity, not pouring
rate, is the factor that controls turbulence in the mold. Metal velocity in those
parts of the mold beyond the sprue can be controlled by appropriate sizing of
the mold passages (gating ratio) in non-pressurized systems, but this does not
100
CHAPTER 6
CONCLUSIONS
The two important aspects on which the entire gating system design depends
are the metal flow characteristics and the casting yield. The flow characteristics
depend on the pouring rate and the casting yield on the size of the gating
system. So, if a high casting yield can be achieved without compromising the
flow characteristics, then the gating system design can be called as a successful
one. Proper design of riser gives less prone to the cavities in the casting.
The program developed for the gating and riser design working
effectively for different riser and gating system combinations. In the design of
riser, riser volume is the volume predicted by solidification time or the feed
metal requirement, which ever gives maximum volume to feed the casting. The
system design. The gating system designed has been given no turbulence and
aspiration problems. End result of the riser and gating system design is high
casting yield. This fact is evident from the case studies made in this work.
101
Hence, we conclude that a functionally and economically sound rigging
system design can be achieved by implementing the design with the aid of
computer.
102
REFERENCES
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:4 AL NNo.........
103 ~r>. xo ..............
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104
18. Mathur, R., 1990, "Computer Aided Design of Gating Systems in
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105 _` • -~ J : _ '_'.~